[llvm] [Offload][cmake] Add GPU test job limit for AMDGPU buildbot cmake cache (PR #146611)

[Shilei Tian via llvm-commits]

shiltian wrote:

> pretty much just found that loading binaries in parallel would crash depending on the machine

This sounds like a loader issue. CC @kzhuravl

https://github.com/llvm/llvm-project/pull/146611